A biogas may be produced through the anaerobic digestion of a material containing biomass. The biogas is typically comprised of 50-75% methane and 25-50% carbon dioxide. Other gases, such as nitrogen, hydrogen, hydrogen sulfide or oxygen may be also present but collectively are unlikely to account for more than 10% of the biogas. Of these other gases, nitrogen is likely to be the largest component. The biogas can be burned directly with oxygen, for example, and so is usable as a fuel. The methane within the biogas can also be concentrated to provide a replacement for natural gas.
Biogas can be produced in an anaerobic digester. The digestion process involves microorganisms, primarily bacteria, which break down or convert the input materials to produce the biogas and an effluent. The process involves a series of bacteria types and processes, primarily hydrolysis, acidogenesis, acetogenesis and methanogenesis.
The composition of the feedstock is important to the biogas generation process. Anaerobic digesters were originally designed primarily for use with cattle manure and sludges. Other feedstock may have a different composition of biodegradable material. In general, simple carbohydrates are easiest to digest whereas large molecules are more difficult to digest. The carbon to nitrogen ratio of the feedstock is also relevant, with a C:N ratio of 20-30:1 being preferred. The moisture content or solids concentration may also vary between feedstocks.
Municipal wastewater, or sewage, is typically treated using an activated sludge process with primary clarification, a biological process train, and secondary clarification. The biological process train may provide oxidation, nitrification and denitrification by way of an anoxic-aerobic-anoxic reactor train. If low nitrate and total nitrogen concentrations are required in the effluent, methanol or another external carbon source is added to the first anoxic reactor, and possibly also to a second anoxic reactor, to enable stable and more complete denitrification. Waste activated sludge, optionally thickened, and primary sludge may be fed to an anaerobic digester at a dry solids (DS) concentration of up to about 4%.
Recently, there has been some research regarding the possibility of treating low strength wastewater (<1000 mg COD/L) by feeding essentially all of the wastewater directly to an anaerobic digester. Low strength wastewaters include, for example, ordinary municipal wastewater or sewage. The primary driver for this research is that, relative to conventional activated sludge treatment, anaerobic treatment offers the possibility of reduced sludge generation, an energy savings resulting from avoiding aerating the mixed liquor, and energy recovery from the biogas. However, in addition to the low chemical oxygen demand (COD) concentration, more than half of the COD in municipal wastewater is in the form of suspended solids, and the water is often cool, perhaps around 10 to 25 degrees C. These factors slow down the kinetics of anaerobic bacteria and the hydrolysis rate of particulate matter, and tends to take key process parameters (such as SRT, organic loading rate and food to micro-organism ratio) outside of the conditions present in working mesophilic digesters or industrial wastewater digesters. Further, the effluent from anaerobic digesters does not typically meet secondary treatment effluent quality standards for solids, nitrogen or phosphorous.